1. Field of the Invention
The present invention relates to an image pickup apparatus, such as a video camera or an electronic still camera.
2. Description of the Related Art
In recent years, a greater number of image pickup apparatuses, such as video cameras or electronic still cameras, have been provided with the zooming function of varying an angle of view.
There are three major zoom systems which will be described in the following paragraphs (a), (b) and (c).
(a) Optical-Zoom System
The optical-zoom system optically varies a photographic angle of view by means of a zoom lens or the like, and is arranged to enlarge or reduce an image by causing a zooming lens group which performs a zooming operation to move along the optical axis by manual operation or by means of a motor (actuator). The advantage of the optical-zoom system is that no great degradation occurs in image quality. The disadvantage of the optical-zoom system is that a large optical system and a driving circuit or mechanism, hence a rise in price, are needed.
(b) Electronic-Zoom System
The electronic-zoom system varies an angle of view by electronic image processing, and is arranged to enlarge or reduce an image by the conversion of the coordinates of a video signal. The advantage of the electronic-zoom system is that the electronic-zoom system is inexpensive and suited for miniaturization. The disadvantage of the electronic-zoom system is that since subsampling and interpolation of pixels, scanning lines and the like are needed, image quality is degraded.
(c) Hybrid-Zoom System
The hybrid-zoom system is a combination of an optical zoom and an electronic zoom. Since the magnification of the optical zoom can be set low and electronic zooming can be implemented by using a high-resolution image pickup element, the hybrid-zoom system can realize small-size, high-magnification zooming means having good characteristics which do not cause a great degradation of image quality.
As is known, a greater number of video cameras have currently been provided with an image-shake correcting function (image stabilizing function). There are three types of image-shake correcting systems: mechanical, optical and electronic, and a purely electronic, image-shake correcting system is particularly suited to a reduction in size and weight because no complex, correcting optical system is needed.
The operational principle of the purely electronic, image-shake correcting system will be described below with reference to FIG. 1.
First, an "image-shake vector" indicative of a motion of an image due to an image shake, i.e., the amount and the direction of the image shake, is detected from a picked-up video signal (Part (a) of FIG. 1). Then, a "cut-out frame" which is required to cut out photographed-image information from an image pickup element or a memory in which an image for one picture is stored is moved according to the detected image-shake vector (Parts (b1) and (b2) of FIG. 1). Finally, the cut-out image is enlarged to the size of a television (TV) picture frame (Part (c) of FIG. 1). Therefore, the "cut-out frame" is smaller than the image pickup area of the image pickup element or the area of the memory in which image data for one picture can be stored.
By continuously performing the above-described operations, it is possible to obtain a video image in which the influence of the image shake is greatly reduced.
The above-described image-shake correcting operation is executed by an image-shake correcting device, such as that shown in FIG. 2. FIG. 2 is a block diagram showing the construction of the image-shake correcting device. In FIG. 2, reference numeral 1 denotes a video signal which is already digitized after having been picked up by an image pickup element. The video signal 1 is an input signal of an image-shake correction detecting system. The image-shake correcting device shown in FIG. 2 includes a filter 2 for extracting from the input signal a signal (representative point) which has a feature in terms of chrominance and luminance in a photographed subject image, and a representative-point memory 3 for storing the representative point. The representative point is obtained by using, for example, a pattern of color information, a luminance peak, a bi-level luminance image or the like. The device shown in FIG. 2 also includes a computing part 4 for detecting a motion of an image from the past representative points and the current representative points, a computation memory 5 for storing a computation result provided by the computing part 4, and a data detecting part 6 for reading computing data from the computation memory 5 and sending it to a microcomputer which will be described later.
The device shown in FIG. 2 also includes a microcomputer 7 for determining the setting of the "cut-out frame" according to the value of the image-shake vector, a field memory 8 for storing the input video signal 1 in an amount corresponding to one picture, and a field memory control part 9 for cutting out part of the video signal stored in the field memory 8 by varying the read-out addresses of the field memory 8 in response to information indicative of the cut-out frame set by the microcomputer 7. Specifically, the field memory control part 9 shifts the cut-out position of an image on the field memory 8 in a direction in which the image-shake vector is cancelled, thereby compensating for a motion of the image. The shown device also includes an electronic-zoom part 10 for electronically enlarging or reducing the cut-out video signal to the size of the TV picture frame. The electronic-zoom part 10 also performs interpolation processing for enlargement. Reference numeral 11 denotes a corrected video output.
The conventional hybrid-zoom system, in which the optical zoom and the electronic zoom are combined, merely uses the electronic zoom as an extension of the optical zoom and is not set so that the disadvantages of both zooms can be compensated for each other. A hybrid-zoom system in which the electronic zoom and the optical zoom are combined in such a simple manner is disclosed in U.S. Pat. No. 4,843,475, U.S. patent application Ser. No. 078,565 (Jun. 17, 1993) and others.
In the conventional electronic image-shake correcting device, since one particular size is preset as the size of the "cut-out frame", the range of image-shake correction is comparatively narrow, and if the magnitude of an image shake exceeds the range, a spatially or temporarily discontinuous video image will be produced.